This application claims the benefit of Korean Patent Application No. 1999-59600, filed on Dec. 21, 1999, under 35 U.S.C. xc2xa7119, the entirety of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device.
2. Description of Related Art
In general, liquid crystal displays are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an internal or external light source.
A typical transmissive LCD device includes a liquid crystal panel and a back light device. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed therebetween. The upper substrate includes a color filter, and the lower substrate includes thin film transistors (TFTs) as switching elements. An upper polarizer is arranged on the liquid crystal panel, and a lower polarizer is arranged between the liquid crystal panel and the backlight device.
The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive LCD device has a transmittance of about 7.4% as seen in FIG. 1. FIG. 1 shows a transmittance (in brightness %) after light passes through each layer of the device. For this reason, the transmissive LCD device requires a high, initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device. However, this presents a problem in that the battery can not be used for a lengthy period of time.
In order to overcome the problem described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light, it is light and easy to carry. Also, the reflective LCD device is superior in aperture ratio to the transmissive LCD device.
FIG. 2 shows a cross-section of a typical reflective LCD device. As shown in FIG. 2, the reflective LCD device includes upper and lower substrates 8 and 10 with a liquid crystal layer 12 interposed. The upper substrate 8 includes color filter layers 4a, 4b and 4c (e.g., red, green, and blue) and a common electrode 6. The lower substrate 10 includes a switching element (not shown) and a reflective electrode 2.
Ambient light 100 passes through the upper substrate 8 and the liquid crystal layer 12 and is reflected on the reflective electrode 2. When electrical signals are applied to the reflective electrode 2 by the switching element, the phase of the liquid crystal layer 12 varies. Then, reflected light is colored by the color filter layers 4a, 4b and 4c and displayed in the form of images.
However, the reflective LCD device is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from the light outdoors. Even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk).
In order to overcome the problems described above, a transflective LCD device has been developed. FIG. 3 shows a conventional transflective LCD device. As shown in FIG. 3, the conventional transflective LCD device includes a gate line arranged in a transverse direction and a gate electrode 52 extended from the gate line 50. A data line is formed in the direction perpendicular to the gate line 50. A source electrode 62 extended from the data line 60 is overlapped with the gate electrode 52.
A drain electrode 64 is formed spaced apart from the source electrode 62. The drain electrode 64 contacts the pixel portions 68 and 70 formed of different materials, via a contact hole 66. The pixel portions have a reflective electrode 68 of substantially non transparent material and a pixel electrode 70 of transparent conducting material. The reflective electrode 68 includes a transmitting hole 72, which can have a rectangular shape. The pixel electrode 70 is larger than the transmitting hole 72 of the reflective electrode 68.
FIGS. 4A to 4D illustrate manufacturing process in cross section taken along line IVxe2x80x94IV of FIG. 3. FIG. 4A shows a gate electrode 52 on the substrate 1. The gate electrode 52 is made of a material chosen from tungsten(W), Chrome(Cr), or aluminum alloy.
FIG. 4B shows a gate insulation layer 80 and the semiconductor layer 82 and source and drain electrodes 62 and 64 stacked in this order.
FIG. 4C shows a protection layer 84 on the source and drain electrodes 62 and 64. The protection layer 84 has a drain contact hole 66 at a corresponding position of the drain electrode 64. The protection layer is made of a material chosen from silicon nitride, silicon oxide, and so on. The pixel electrode 70 is formed on the protection layer 84. The pixel electrode 70 has indium tin oxide and contacts the drain electrode 64 via the drain contact hole 66.
FIG. 4D shows formation of a reflective electrode 68. An insulation layer 86 of benzocyclobutene (BCB) is deposited on the pixel electrode 70 and patterned to expose a portion of the pixel electrode 70 near the drain contact hole 66. Afterwards, the reflective electrode 68 is formed on the insulation layer 86.
FIG. 5 schematically shows a transflective LCD device in cross section. The portion of the transmitting hole, the pixel electrode and the reflective electrode are emphasized in the drawing.
The transflective LCD device in FIG. 5 is operable in transmissive and reflective modes. First, in reflective mode, the incident light 110 from the upper substrate 106 is reflected on the reflective electrode 68 and directed toward the upper substrate 106. At this time, when electrical signals are applied to the reflective electrode 68 by the switching element (not shown), phase of the liquid crystal layer 100 varies and thus the reflected light is colored by the color filter 104 and displayed in the form of images.
Further, in transmissive mode, light 112 generated from the backlight device 102 passes through portions of the pixel electrode 70 corresponding to the transmitting holes 72. When the electrical signals are applied to the pixel electrode 70 by the switching element (not shown), phase of the liquid crystal layer 100 varies. Thus, the light 112 passing through the liquid crystal layer 100 is colored by the color filter 104 and displayed in the form of images.
As described above, since the transflective LCD device has both transmissive and reflective modes, the transflective LCD device can be used without regard to the time of day (e.g., noon or dusk). It also has the advantage that it can be used for a long time by consuming low power.
FIG. 6 is an enlarged view of xe2x80x9cAxe2x80x9d portion of FIG. 5. Distance between the upper surface of the pixel electrode 70 and the upper surface of the reflective electrode 68 is designated as xe2x80x9cdxe2x80x9d, which is caused mainly by the insulation layer 86. Since an equipotential surface is formed along surfaces of the electrodes, distortion occurs in the electric field at the interface portion xe2x80x9cFxe2x80x9d of the two electrodes 68 and 70.
FIG. 7 illustrates a simulation graph showing equipotential lines and the direction of liquid crystal molecules in case of adopting the insulation layer 86 of 2 xcexcm. The simulation result reveals that the electric fields are much in disarray near the interface between the reflective and pixel electrodes 68 and 70. The liquid crystal molecules in the transmitting hole 72 do not have a uniform arrangement direction 85, which is mainly affected by the equipotential line 67. In a case of dark lighting conditions, since the arrangement of the liquid crystal molecules in the transmitting hole or portion 72 does not have symmetry and uniformity, the light from a back light device which leaks out though an optimized polarizer is adopted, resulting in a lowering of the contrast ratio.
Accordingly, the present invention is directed to a transflective liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
It is an object of the invention to provide a transflective LCD device having an improved contrast ratio.
In accordance with the purpose of the invention, as embodied and broadly described, in one aspect the invention provides an array substrate for a transflective LCD device including a substrate; a thin film transistor having gate, source and drain electrodes on the substrate; a protection layer on the thin film transistor and the substrate; a pixel electrode on the protection layer, the pixel electrode contacting the drain electrode of the thin film transistor; a reflective electrode contacting the drain electrode, the reflective electrode having a first transmitting hole; an insulation layer having a thickness between the pixel and reflective electrodes, the insulation layer having a second transmitting hole corresponding to the first transmitting hole; and wherein a distance between upper surfaces of the reflective and pixel electrodes is less than 0.5 micrometers.
In another aspect, the invention includes an array substrate of a transflective liquid crystal display device, including: a substrate having a switching region and a display region; a thin film transistor on the switching region of the substrate, the transistor having gate, source, and drain electrodes; a drain supplementary electrode extended from the drain electrode to the display region of the substrate; a protection layer covering the transistor and the drain supplementary electrode and having a first drain contact hole exposing a portion of the drain supplementary electrode; a pixel electrode on the display region of the substrate, the pixel electrode contacting the drain supplementary electrode via the first drain contact hole; an insulation layer on the pixel electrode and covering the transistor, the insulation layer having a second drain contact hole exposing a portion of the drain electrode; and a reflective electrode on the insulation layer, the reflective electrode contacting the drain electrode via the second drain contact hole, having a transmitting hole exposing the insulation layer over the display region of the substrate, and having a thickness such that sum of the thickness of the insulation layer and the thick of the reflective electrode is substantially the same as the thickness of the pixel electrode.
The upper surface of the reflective electrode and that of the pixel electrode can be in the same plane.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and together with the description serve to explain the principles of the invention.